In the pharmaceutical industry, an optical particle analyzer is frequently used to analyze samples including particles of biological material dispersed in a water-based carrier. As the optical properties of the particles in such samples are similar to those of the carrier, the analysis of such samples is particularly challenging. Not all of the particles in such samples may be detectable by the optical particle analyzer, owing to detection-sensitivity limitations. To achieve reliable and repeatable results for the analysis, the optical particle analyzer must be properly calibrated and periodically validated by using a particle standard that optically approximates such samples. In particular, it is highly desirable that the fraction of particles detected in such samples is substantially the same when the samples are analyzed at different times or locations, with the same or different optical particle analyzers.
Unfortunately, samples including particles of biological material dispersed in a water-based carrier are, generally, unstable and cannot be practically used as particle standards. Therefore, a particle standard that may serve as a stable optical surrogate for such samples and a method of calibrating or validating an optical particle analyzer by using the particle standard are required.
However, most conventional methods of calibrating or validating an optical particle analyzer involve the use of a particle standard including particles having optical properties dissimilar to those of a carrier in which the particles are dispersed. For example, methods involving the use of a particle standard including polymer particles having a refractive index significantly different from that of a water-based carrier in which the particles are dispersed are described in U.S. Pat. No. 5,728,582 to Taki, et al., issued on Mar. 17, 1998, in U.S. Pat. No. 4,704,891 to Recktenwald, et al., issued on Nov. 10, 1987, and in U.S. Pat. No. 4,331,862 to Ryan, issued on May 25, 1982, which are incorporated herein by reference. For another example, methods involving the use of a particle standard including stained particles having fluorescence or transmission properties significantly different from those of an unstained carrier in which the particles are dispersed are described in U.S. Pat. No. 6,542,833 to Nygaard, issued on Apr. 1, 2003, which is incorporated herein by reference, in U.S. Pat. No. 5,728,582, and in U.S. Pat. No. 4,704,891.
As the particles in the particle standards used in such methods are easily detected by the optical particle analyzer, the particle standards do not challenge the detection sensitivity of the optical particle analyzer. Optical particle analyzers having different detection sensitivities are, typically, able to detect substantially all of the particles in the particle standards; however, they may detect significantly different fractions of particles in samples including particles of biological material dispersed in a water-based carrier, leading to unreliable and unrepeatable results for the analysis of such samples.
Furthermore, most conventional methods of calibrating or validating an optical particle analyzer, such as those mentioned heretofore, and those described in U.S. Pat. No. 6,074,879 to Zelmanovic, et al., issued on Jun. 13, 2000, and in U.S. Pat. No. 5,747,667 to Sadar, issued on May 5, 1998, which are incorporated herein by reference, require that substantially all of the particles in a particle standard be detected by the optical particle analyzer. Although, as mentioned heretofore, this requirement may be easy to fulfill for a particle standard including particles having optical properties dissimilar to those of a carrier in which the particles are dispersed, it may be difficult or impossible to fulfill for a particle standard that optically approximates samples including particles of biological material dispersed in a water-based carrier.